I modified the code in the Replacement code for YouTube to the dimensions that I like.
Go to the AME CP in your Admin CP, then Display Definitions>>"You Tube
"Automatically youtubes your entries!>>Media Definition Code>>Replacement Code
Here's my code:
Code:
<object width="500" height="412">
<param name="movie" value="http://www.youtube.com/v/$p1&ap=%2526fmt%3D18&fs=1"></param>
<param name="allowFullScreen" value="true"></param>
<embed src="http://www.youtube.com/v/$p1&ap=%2526fmt%3D18&fs=1" type="application/x-shockwave-flash" allowfullscreen="true" width="500" height="412" wmode="transparent"></embed></object>